Thermal treatment systems are used, for example, to ablate liver metastatic tumors, treat benign prostatic hyperplasia, ablate cardiac tissue for the prevention of arrhythmia, heat corneal tissue to correct myopia, occlude the Fallopian tubes and for other therapeutic purposes.
Conventional thermal treatment systems and methods apply constant, or substantially constant, electromagnetic energy until a therapeutic combination of tissue temperature and accumulated treatment time is attained. However, relatively few conventional systems have the ability to monitor progress of a thermal procedure in real-time. It is thus difficult to determine when an intended outcome has been achieved or, more importantly, to regulate energy output in anticipation of an intended outcome, to prevent overshooting of an endpoint.
Minimally-invasive and non-invasive thermal treatment modalities are advantageous because they reduce the risk of infection and decrease recovery times. However, the benefits of non-invasive treatment modalities can only be recognized when monitoring is also performed non-invasively. Some current systems with the ability to non-invasively monitor thermal tissue changes utilize optical techniques, such as near infrared tomography or birefringence techniques. For example, optical opacity or birefringence of corneal tissue may be monitored during a thermal keratoplasty operation to detect the sudden onset of collagen shrinkage that occurs around 60° C. Nonoptical techniques, including electrical impedance imaging and permittivity imaging, may also be used to non-invasively monitor changes in tissue. However, near infrared tomography, electrical impedance imaging and permittivity imaging require several seconds or minutes to create an image, which is typically too long for real-time monitoring of tissue treatments.
In addition to optical and electrical changes, absorption of electromagnetic energy by tissue causes a number of other phenomena. For example, molecules that absorb electromagnetic energy may experience increased rotation and/or vibration. Such energy may dissipate as heat into surrounding tissue where it can cause expansion of liquid in the tissue, and contribute to the formation of an acoustic pressure wave that propagates away from an affected region.
In U.S. Pat. No. 6,694,173, Bende et al. disclose use of acoustic pressure waves to monitor progress of a laser photoablation procedure. The pressure wave is induced by the ablating laser, and differences between tissue layers are detected and stratographically mapped as the tissue layers are ablated. Thus, the '173 patent monitors tissue removal rather than compositional changes within a given target tissue.